1. Field of the Invention
The present invention concerns a process for producing aryloxy-substituted phosphazene derivatives, for which industrial application use has now been developed, such as flame retardants, lubricating oils, electric insulators, hydraulic oils or starting materials therefor.
2. Description of the Prior Art
For oxyphosphazene derivatives obtained by alkoxylation and/or aryloxylation of chlorophosphazene oligomers having cyclic or chain structure ##STR3## where n is an integer of 3 or greater, particularly, a cyclic chlorophosphazene oligomer ##STR4## where n is 3 or 4, or chlorophosphazene oligomer mixtures comprising them as the main ingredient, or polymeric compounds resulted from their condensation, application uses as various industrial materials have now been developed owing to their excellent heat resistance, chemical resistance, lubricity, electric insulation property or chemical stability.
Various methods used for producing these materials, i.e., substitution of chlorine atoms bonded to phosphorus atoms in the phosphazene skelton (hereinafter referred to as "active chlorine atoms") with an aryloxy groups represented by the formula ##STR5## (where R' represents hydrogen or a substituent other than after mentioned maleimido group, for instance, an alkyl group, an alkoxy group or a halogen atom) as illustrated in "Phosphorus-Nitrogen Compounds", pp. 150-155, written by H. R. Allcock, published by Academic Press Inc. (1972). Then, the known methods of substituting active chlorine atoms with aryloxy groups (phenoxy group, for instance) may generally be classified into methods of reacting active chlorine atoms, with:
(1) phenol in the form of an alkali metal salt,
(2) phenol as it is, using a hydroxide or carbonate of alkali metal as an acid acceptor,
(3) phenol while using a tertiary amine as an acid acceptor, and
(4) phenol under the presence of an acid acceptor using a phase transfer catalyst (U.S. Pat. No. 4,600,791, July 15, 1981).
Further, the production process wherein the aryloxy group is a 4-maleimidophenoxy group represented by the formula ##STR6## is described in U.S. Pat. No. 4,550,177 or "Journal of Polymer Science", Polymer Chemistry Edition, vol. 22, pp. 927-943 (1984), etc.
However, the method (1) described above takes an extremely long period of time to completely replace the all active chlorine atoms with the aryloxy groups.
In the method (2) described above, a side reaction is liable to occur, in which by-produced water reacts with the active chlorine atoms or with substituted oxy groups already bonded to the phosphorus atoms in the phosphazene skelton to cause P--OH bonding.
In the method (3) described above, it is generally difficult to completely replace the active chlorine atoms entirely with the phenoxy groups, although there is an exception such as the case of p-nitrophenol.
Then, the method (4) described above requires an expensive catalyst in a considerably great amount and a relatively long time for the completion of the reaction. Further, since water is used in this reaction, there is a great possibility that by-products having --OH groups are resulted in the case of using easily hydrolyzable material (for example, linear chlorophosphazene oligomer) as the starting material.
Further, since all of the reactions from (1) to (4) are conducted under heating, in the case where a phosphazene oligomer, which has substituents easily polymerizable by heating (for example, an aliphatic group having unsaturated bonding), is used as the starting material, it is inevitable to form by-products by polymerization. Furthermore, in these reactions, it is difficult to control the substitution of the active chlorine atoms with the aryloxy group to desired step.
While on the other hand, in a case where the aryloxy group is a 4-maleimidophenoxy group represented by ##STR7## 4-maleimidophenol is not directly reacted in the method of the literature described above but in extremely complicated procedures including a plurality of steps such as reaction of hexachlorocyclotriphosphazene (hereinafter simply referred as "3PNC") with 4-nitrophenolate or with 4-nitrophenolate and phenolate, followed by hydrogen reduction of the nitro group under pressure, maleamidation of the resultant amino group and formation of the aimed product by the dehydrating ring closing. Further by this method, the final yield is low, and it can not be acceptable from the economical point of view.
Further, the present inventors have previously offered a method of reacting 4-maleimidophenol directly with a chlorophosphazene oligomer in order to overcome the foregoing drawback (U.S. patent application Ser. No. 090,368), but it requires still a long period of time for the completion of the reaction.